High porous media

Miscellaneous high-pressure NMR studies of inorganic systems High-pressure NMR studies using sapphire tubes Various high-pressure studies of polymers and complex liquids High-pressure studies of liquids confined to porous media High-pressure NMR studies of solids... 

Direktor, L.B., Zaichenko, V.M., Maikov, I.L., et al. Theoretical and experimental smdies of hydrodynamics and heat exchange in porous media. High Temp. 48, 887-895 (2010)... 

Nilsson, O., Riischenpohler, G., Gross, J. and Fricke, J., Correlation between thermal conductivity and elasto-mechanical properties of compressed porous media. High Temperatures - High Pressures, 1989. 21 p. 267. 

In situations where a low concentration of suspended solids needs to be separated from a liquid, then cross-flow filtration can be used. The most common design uses a porous tube. The suspension is passed through the tube at high velocity and is concentrated as the liquid flows through the porous medium. The turbulent flow prevents the formation of a filter cake, and the solids are removed as a more concentrated slurry. 

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

Porous Media Packed beds of granular solids are one type of the general class referred to as porous media, which include geological formations such as petroleum reservoirs and aquifers, manufactured materials such as sintered metals and porous catalysts, burning coal or char particles, and textile fabrics, to name a few. Pressure drop for incompressible flow across a porous medium has the same quahtative behavior as that given by Leva s correlation in the preceding. At low Reynolds numbers, viscous forces dominate and pressure drop is proportional to fluid viscosity and superficial velocity, and at high Reynolds numbers, pressure drop is proportional to fluid density and to the square of superficial velocity. 

FIGURE 23.3 A solution of a bimodal mixture of low molecular weight components (black) and high weight components (white) in contact with a porous medium (a) Low and (b) high concentrations. 

Aluminum foam can be used as a porous medium in the model of a heat sink with inner heat generation (Hetsroni et al. 2006a). Open-cell metal foam has a good effective thermal conductivity and a high specific solid-fluid interfacial surface area. 

At high Reynolds numbers (high turbulence levels), the flow is dominated by inertial forces and wall roughness, as in pipe flow. The porous medium can be considered an extremely rough conduit, with s/d 1. Thus, the flow at a sufficiently high Reynolds number should be fully turbulent and the friction factor should be constant. This has been confirmed by observations, with the value of the constant equal to approximately 1.75 ... 

Nonadsorptive retention of contaminants can also be beneficial. For example, oil droplets in the subsurface are effective in developing a reactive layer or decreasing the permeability of a sandy porous medium. Coulibaly and Borden (2004) describe laboratory and field studies where edible oils were successfully injected into the subsurface, as part of an in-situ permeable reactive barrier. The oil used in the experiment was injected in the subsurface either as a nonaqueous phase liquid (NAPL) or as an oil-in-water emulsion. The oil-in-water emulsion can be distributed through sands without excessive pressure buildup, contrary to NAPL injection, which requires introduction to the subsurface by high pressure. 

Although such studies are in their early stages, this example clearly demonstrates that we have the measurement tools to investigate the complex interaction of hydrodynamics and chemical kinetics in the complex porous medium represented by a fixed bed. Looking to the future, we may expect experiments of this nature to demonstrate how a catalyst with intrinsic high selectivity can produce a far wider product distribution when operated in a fixed-bed environment as a result of the spatial heterogeneity in hydrodynamics and hence, for example, mass transfer characteristics between the inter-pellet space within the bed and the internal pore space of the catalyst. 

Figure 25.3 The hydraulic gradient, defined as the slope of the (unconfined) groundwater table, Swl = A/i /Ax, is a measure for the horizontal pressure gradient that drives the flow through a porous medium from high to low pressure.

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

The retardation factors of the four radioelements for four hypothetical HLW compositions were derived using the prediction equations. (The retardation factor is the ratio of the solution velocity to the radioelement velocity in a system of solution flow through a porous medium and increases linearly with Kd.) The four hypothetical HLW solutions broadly represented dilute/non-complexed, dilute/complexed, concentrated/noncomplexed, and concentrated/complexed HLW. Dilute waste had low concentrations while concentrated waste had high concentrations of Na+, NaOH, and NaAlO,. Non-complexed waste had no HEDTA or EDTA while complexed waste had 0.1M HEDTA/0.05M EDTA. 

The distribution coefficient Kj (Equation 2) is defined as the volume fraction of pores, in a stationary phase, which is effectively permeated by a solute of a given size. V0 is the interstitial volume of the porous medium, measured by the elution volume of a high molar mass solute that is totally excluded from the matrix pores. Ve is the elution volume of the product of interest. Vs represents the total solvent volume within the pores, available for small solutes. 

The reduction of the thickness of the flow channel, as discussed earlier, is equivalent to introducing more surface area per unit volume of medium. High surface areas inhibit all flow, including natural convective flow. One can increase relative surface areas by going to thinner tubes or channels, or by using a fine granular or porous support medium. Both approaches are used in electrophoresis as discussed in a subsequent chapter. 

Attention will here be restricted to two-dimensional steady flow in a rectangular porous medium -filled enclosure which is, in general, inclined at an angle to the vertical. One wall of the enclosure is kept at a uniform high temperature and the opposite wall is kept at a uniform low temperature. The o her two walls of the enclosure are assumed to be adiabatic, i.e., it is assumed that no heat is transferred into or out of these walls. The situation is, therefore, as shown in Fig. 10.27. 

In the petroleum industry, HC1 is routinely injected into carbonate formations in order to improve oil or gas production. It is known that the porous medium is not etched uniformly by the reactive fluid but that unstable dissolution patterns consisting of highly ramified, empty channels are formed. The channels are commonly called wormholes. As soon as a wormhole pattern develops, all the fluid will flow through it. Any local increase in the flow rate results in an increase in the local dissolution rate. A piece of porous medium in which a wormhole pattern has been created can be considered as composed of two parts the first part (wormholes) of very large permeability, and the second part keeping its original permeability. 

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